DataTrigger to a control's Validation.HasError

Question

I have a user control which uses the INotifyDataErrorInfo interface and it turns red when it has errors, inside of this user control I put a TextBlock and the following DataTrigger doesn't seem to work:

<TextBlock Text="{Binding DurationText}"
            Grid.Row="1">
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding ElementName=ActivityUserControl, Path=(Validation.HasError)}"
                             Value="True">
                    <Setter Property="Foreground"
                            Value="White">
                    </Setter>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

I made sure that the x:Name of my user control is correct (ActivityUserControl), the user control itself turns red when it has errors but the TextBlock's DataTrigger doesn't work (text stays black).

Answer 1

Your should set the Value of the DataTrigger to begin with:

<DataTrigger Binding="{Binding ElementName=ActivityUserControl, Path=(Validation.HasError)}"
             Value="True">
...

You should also make sure that the ActivityUserControl is in the same namescope as the TextBlock and that it acually contains some validation errors.

You can solve the namescope issue by binding to the parent UserControl using the RelativeSource property:

Binding="{Binding Path=(Validation.HasError), 
    RelativeSource={RelativeSource AncestorType=UserControl}}"